This invention relates to an atomic absorption photometer and in particular to an atomic absorption photometer containing power source circuits for its light sources.
Atomic absorption photometers use a hollow cathode lamp (HCL) as a source of light for observing atomic absorption. The HCL is a discharge tube, adapted to start discharging if a DC voltage above its starting voltage is applied thereto and to generate light by exciting the gas which is sealed inside. Once the discharge is started, it can be continued and the light-emitting condition can be maintained by a voltage which is lower than the starting voltage while it should still be above its discharge maintaining voltage. The starting voltage is 170-350 V and the discharge maintaining voltage is 120-240 V but these values vary, depending on the kind of the sealed element, individual characteristics of the lamp and the age of the lamp. Although the discharge maintaining voltage is lower than the starting voltage, the difference therebetween is small, and the starting voltage is usually applied continuously to maintain the discharge thereafter. The discharge current is about 4-40 mA.
As the power source of an HCL, a commercial power source is usually used after its voltage is changed by means of a transformer for commercial frequencies and the current is subjected to rectifying and smoothing processes.
In order to obtain stable data in atomic absorption measurements, it is important to stabilize the brightness of the HCL. For this purpose, it is necessary to control the discharge current, and it has been common to make use of a steady current circuit which is a combination of an operational amplifier and a transistor or a field effect transistor (both being herein referred to as "the current-steadying transistor").
In many situations in atomic absorption photometry, a heavy hydrogen (deuterium) lamp (D2L) is also used as the light source for a background correction, that is, for making corrections on undesired absorption. In order to start the discharge of a D2L, a DC current is passed first through its filament to heat it and a DC voltage greater than its starting voltage is applied after a specified length of time. Immediately thereafter, the applied voltage is lowered to a level which is still above its discharge maintaining voltage to maintain the discharge so as to maintain the lamp in the light-emitting condition. For a D2L, the starting voltage is 350-400 V and the discharge maintaining voltage is 75-95 V but these values also vary, depending on individual characteristics and the age of the lamp. The discharge current is usually about 300 mA. Once the discharge is started, the voltage for heating the filament is also lowered in order to reduce the heat production from the lamp and hence to improve its useful lifetime. The voltage and the current through the filament are, for example, 2.5 V and 4A at the time of starting the discharge and 1.0 V and 1.8A after the lamp is lit. As the power source of an D2L, it has also been customary to use a commercial power source after varying its voltage by means of a transformer for commercial frequencies and subjecting the current to rectifying and smoothing processes. In order to obtain stable data in atomic absorption photometry, it is also important to stabilize the brightness of the D2L. For this purpose, it is necessary to control its discharge current and it has been common also to make use of a steady current circuit which is a combination of an operational amplifier and a current-steadying transistor.
The prior art technology described above has several problems. With a transformer for commercial frequencies; for example, the input voltage and the output voltage change in a mutually proportional relationship and hence its output voltage becomes lower if the voltage of the commercial power source serving as its power source becomes lower. For this reason, the prior art power source for the lamp has been designed by taking into consideration in particular the possibility of the input voltage dropping to a lower level. In other words, the output voltage of transformers for commercial frequency has been set such that a sufficiently steady current operation would be possible even when the input voltage has dropped to the lowest level within a specified range and the discharge maintaining voltage of the lamp was at its highest level.
In a power source circuit, the voltage (Vsi) applied to its steady-current circuit is determined by the following formula: EQU Vsi=Vmo-Vlm (1)
where Vmo represents the output voltage of a discharge maintaining power source and Vlm represents the discharge maintaining voltage of the lamp. Since the power source is designed for the case of a lowest input voltage, as explained above, a somewhat higher voltage is applied to the steady-current circuit when a standard input voltage is applied to the power source circuit. If the voltage applied to the power source circuit is at its highest level, the voltage applied to the steady-current circuit becomes undesirably high. This is particularly so if the discharge maintaining voltage of the lamp is low.
In the steady-current circuit, the voltage (Vti) applied to its current-steadying transistor is nearly equal to the voltage applied to the steady-current circuit, that is: EQU Vti.apprxeq.Vsi (2)
In other words, the voltage applied to the steady-current circuit is nearly entirely applied to the current-steadying transistor. Thus, the power (Wt) consumed by the current-steadying transistor is give as follows: EQU Wt.apprxeq.(Vmo-Vlm).times.(Discharge current of the lamp) (3)
Because the power Wt given by (3) eventually becomes heat, the current-steadying transistor keeps generating heat at a significant rate even if the input voltage is at a standard level. Since the generation of heat not only implies a waste of useful energy but also affects the reliability of the current-steadying transistor adversely, it is necessary to make use of a large transistor and a heat radiator, but this brings in an additional problem that the steady-current characteristics become unstable during the transitory period before a thermal equilibrium is reached. This is a particularly serious problem when the input voltage approaches the upper limit of the specified range.
The HCL, in particular, is usually designed in accordance with the largest values of the discharge starting and discharge maintaining voltages for all of the target elements to be used for the atomic absorption photometer. Thus, the heat production from the current-steadying transistor according to Formula (3) becomes particularly large while the HCL for an element with a low discharge maintaining voltage is lit, making the aforementioned problems even more serious.
Since the discharge starting voltage of the D2L is a little higher than that of the HCL, it may be tempting to use a common power source for both of them. As explained above, however, variations are large in the output voltage of a discharge starting source for an HCL using a transformer for commercial frequencies. When the output voltage of the discharge starting source for the HCL is low and the starting voltage of the D2L is high, in particular, there is a possibility that the D2L may fail to start the discharge. For this reason, it has been customary to provide separate power sources for these two lamps.
A constant voltage source is used for the filament of a D2L and since it may be directly connected to the filament, there is no problem of heat generation by the current-steadying transistor. If the input voltage is outside a specified range, however, emission of light from the D2L may become unstable or the light may even be extinguished. If the voltage applied to the filament is to be reduced after the discharge is started, furthermore, there are two voltage levels that are required. This means that two kinds of secondary coil for the transformer must be provided and that a switch capable of passing a large current for the filament is required.
If the product is to be internationally marketed, furthermore, it must be usable with different commercial power voltages such as AC100 V, AC 120 V, AC220 V and AC240 V. If a single power source for light source is to be directly used under all these different circumstances, a very large current-steadying transistor and a heat radiator will have to be selected for the aforementioned reason of large heat production. According to a prior art technology, terminals with suitable coils were provided on the primary side of the transformer for commercial frequencies and a commercial power source was connected to one of the terminals so as to switch among different voltages. This, however, gives rise to additional problems such as the cost of a switching device and the time it takes to carry out the switching.
The effective sectional area of the core of a transformer is inversely proportional to its operating frequency. Since transformers for commercial frequencies operate at a relatively low frequency such as 50 and 60 cycles per second, the effective sectional area of their core must be relatively large. This means that such a transformer is bulky and heavy and prevents the atomic absorption photometers incorporating it from being made compact. Transformers for commercial frequencies require many metallic materials such as silicon steel plates and copper wires. If a transformer for commercial frequencies is used, furthermore, a relatively large current-steadying transistor and radiator are required, as explained above. This has been one of the reasons for the increased cost of an atomic absorption photometer.
Because the general specification for current and voltage applies to power sources for the digital and analog circuits within an atomic absorption photometer, it is relatively easy to form a switchable power source and there are many such sources that are commercially available. For power source for a light source, however, there has not been any which was commonly available and it has not been possible to dispense with transistors for commercial frequencies.